TRIBOEMISSION AND BOUNDARY FILM FORMATION

In order to reduce CO2 emissions and thus limit global warming we need to reduce energy consumption. One way to do this is to make the machines that we use in everyday life, ranging from car engines to washing machine motors and bearings, more efficient. This is particularly important since there is a huge rate of growth in the use of machines in countries such as China and India as these become more prosperous. There are several strategies for increasing machine efficiency but one of the most effective is to reduce mechanical friction. So far this has been done mainly by using lower viscosity lubricants, which have less friction drag. However, this approach is reaching the end of its usefulness since, it also leads to thinner fluid films between rubbing surfaces, which eventually results in high wear as well as even more friction. The solution now lies in improving the performance of surfaces films, which can protect components and reduce friction regardless of lubricant viscosity. These are called boundary films, and must be made to form more quickly and durably, and give lower friction. This is currently impossible since, despite a century of research and widespread commercial use, there is inadequate understanding of the mechanisms by which they form.

The biggest gap in our understanding concerns the way that the rubbing process stimulates film formation. When solids are rubbed together actual contact occurs only at a few high spots on the surfaces. The conditions at these contact points are extremely severe, with very high local stresses which plastically deform the rubbing surfaces. Under such conditions, a phenomenon called "triboemission" occurs; i.e. fundamental particles such as electrons, ions and photons are ejected from the surfaces. These energetic particles promote a series of chemical reactions in the lubricant present that leads, ultimately, to the formation of protective boundary lubricating films. These particles can also have harmful effects such as causing lubricant film degradation on computer hard drives.

In order to improve boundary film formation we need to understand triboemission and its effect on lubricants. Unfortunately these processes occur between a pair of rubbing surfaces, where it is difficult to see and measure. Furthermore, particles emitted react almost instantly with the lubricant present, and are obscured by the competing influences of frictional heating and extreme pressure. This makes research on triboemission very challenging to carry out, which is why we currently know so little about it.

The research group which I aim to build will develop and apply a series of novel experimental techniques to study triboemission and to monitor its impact on lubricants and boundary film formation. The key is to look at each stage of the emission and film formation process and link these together. Particle detection apparatus will be built and incorporated into friction testing equipment. Thermal mapping will be used to distinguish triboemission from other causal factors, while fluorescence imaging (currently used mainly in biomedical applications to study molecule mobility) will help track transient reaction species in the lubricant. Additionally, the application of scanning probes will pioneer the mapping of emission and allow correlation with surface properties. In this way, the series of interactions that occur between lubricant and environment will be unravelled. With industrial support, this understanding will be used to design enhanced surface and lubricant combinations. The result will be improved friction performance and, in certain critical applications, protection of the lubricant from degradation.

This summary has focussed on engineering contacts but triboemission is also believed to play a decisive role in the lubrication of bio-contacts and micro-contacts, where friction is a significant factor in performance. These applications will also be studied in my research.

Although this research is fundamental in nature, its implications are potentially far reaching. It is estimated that 30% of global energy consumption is consumed wastefully in friction [1] and boundary friction constitutes a significant and growing proportion of this figure. Therefore, the most important impact of my research to enhance boundary lubrication and hence improve machine efficiency will be a reduction in energy consumption and thus CO2 emission. There are then obvious implications of helping to limit global warming and the consequent improvements to our future quality of life. There is also the more tangible effect of increased competitiveness of lubricant and additive companies within the UK. As with the majority of tribological advances, the increase in machine efficiency arising from this work is likely to progress incrementally. However, the generic nature of lubrication means that improvements can be implemented across a vast range of applications, and therefore the overall impact will be magnified. The attention this project has received from Shell and BP Castrol (see letters of support), suggests that this impact is real. Furthermore, their involvement ensures the effective and global propagation of these advances.

A smaller scale and more direct impact of my research will come from the commercialisation of the measurement systems that will I design. These systems will be used to analyse boundary lubrication and will be a valuable tool for companies seeking to understand how their specific additives, lubricants and surfaces function. Commercialisation will proceed under licence to PCS instruments, a company that currently supplies equipment to all the major oil companies [2]. This process should bring revenue and employment to The College, lubricant companies and equipment manufacturers.

A section of my research aims to elucidate some of the mechanisms that affect the interface between prosthetic implant and human bone. As people live longer and our population ages, effective and long lasting joint replacement is increasingly important. Improvements made to implant surfaces and materials based on the finding from my research, could, potentially lead to fewer cases of conditions such as aseptic loosening, and to a healthier population. Again, even small advances, if they are implemented in such a generic area, result in widespread improvement. It is therefore crucial that the Tribology Group, of which I am part, works with The London Implant Retrieval Centre [3] and has links with Charing Cross Hospital and several hip joint manufactures. This network will be used to ensure that benefits arising from my findings reach areas of impact.

It should also be noted that the methods I will develop to analyse film formation, will also be applied to study and limit degradation of the critically important lubricant layer on the surface of computer hard disks. The improvement in lubricant behaviour that I am seeking should lead to a further reduction in disk-head distances, and the potential increase in memory storage density.

As well as maximising the impact of current research, it is clearly important to encourage the next generation to follow careers in this area, so that they will subsequently work to improve our future state of health, wealth and culture. To do this, I will continue working with the Institute of Engineering and Technology and the Institute of Mechanical Engineers to coordinate outreach programmes that encourage secondary school pupils to engage with tribology, and hopefully peak their interest in research.

[1] Williams, J. A., Engineering Tribology, Cambridge
[2] www.pcs-instruments.com
[3] www1.imperial.ac.uk/surgeryandcancer/divisionofsurgery/clinical_themes/musculo/retrieval